On packet based information transfer networks, such as for example the Internet, IP local area networks or the like, several modes of information transfer are found. These modes may be classified into three categories as a function of the number of senders and of receivers engaged in this transporting. Firstly there is point-to-point transporting (or “unicasting”) which allows a sender to despatch an information packet destined for a single receiver identified by his address on the network. This is the mode of transport used by the most popular protocols on the Internet network such as the HTTP web page transfer protocol (“Hypertext Transfer Protocol”) or the File Transfer Protocol (FTP). Another mode of transport involves a sender transporting a packet in broadcasting mode. In this mode, the packet sent by the sender is sent to all the nodes of the network. This mode is not available on the Internet but is found on local area networks. The third mode involves a sender or a group of senders transporting a packet to a group of receivers, in a multipoint mode of transport (or “multicasting”). In this mode the packets are sent to an address called the multicast address and will be forwarded to all the recipients belonging to the transmission group. A client that joins a transmission group will be said to subscribe to the group and a client that leaves the group will be said to desubscribe from the group.
The multicast mode is used in practice to save intermediate bandwidth in the network when a source sends data to a group of recipients. Specifically, in this case, the use of a unicast mode of transport implies that the data are despatched as many times as there are recipients. This mode brings about the duplication of the packets over the parts of the network that are common to the paths between the source and the various recipients. On the other hand multicast makes it possible to despatch the data just once, these data being duplicated on the routers of the network, as a function of the paths leading to the recipients belonging to the transmission group. FIG. 1a illustrates the transmission of a data packet (P) sent by a node “S”, the information source, to nodes “A”, “B” and “C”. It is seen that the packet “P” is duplicated three times between the node “S” and the router “R1”, twice between the routers “R1” and “R2” in the case of the unicast transfer and is not duplicated in the case of the multipoint transfer illustrated by FIG. 1b. In this case, a single packet “P” is sent by the source “S”, the router “R1” knows that the packet must be retransported on two branches out of three towards the node “A” and the router “R2” which itself transports it to the clients “B” and “C”, the members of the group. The packet is not despatched towards the nodes “D” and “E” that are not members of the transmission group.
A local area network generally comprises a gateway linking the local area network proper and the exterior network, generally the Internet. To this gateway are connected, according to several possible technologies such as Ethernet, IEEE 1394 or technologies for wireless connection by radio, various local appliances. These appliances may access the exterior network via the gateway operating as router between the local area network and the exterior network. When a local appliance, the client, wishes to join a multipoint data transmission group, it subscribes to the multicast address, for example according to the IGMP Internet group management protocol, this protocol being known under the reference “RFC 3376” at the IETF (Internet Engineering Task Force”. Following this subscription, the node is recognised as a member of the transmission group and the packets corresponding to this stream, and transmitted in multipoint mode, are routed from the Internet, via the gateway, to this client.
It may be that over the local area network, depending on the technology used, the multicast is not always performed dependably. For example, in the case where the local area network is a wireless network operating according to a protocol from the 802.11 family in version a, b or g, the packet transported is tested to see whether it is intact and packets that are not intact are discarded but not retransported. They are lost.